I like to knit from the top so I can try the sweater on as it progresses. I find it easier to get the sleeve and body length right knitting from the top, and I love the fact that I can easily lengthen or shorten the body or the sleeves if I want to.
I also design all of my own sweaters and/or adapt designs so that I can knit them top-down, in the yarn I want to use, to fit the body of the intended wearer. If I'm going to spend many hours of my life making a sweater, I want it to look and feel good.
Someone recently sent me email asking me to teach her how I knit the way I do. I've idly considered writing a "designing sweaters to be knit from the top-down so they fit nicely" tutorial, and now I'm going to actually try to do it.
The first step when designing a sweater is to catch your intended wearer (hereafter referred to as "victim") and measure her body.
I'm going to be heretical here for a moment and say that, if you are the intended wearer, you can measure yourself. You don't need to wait for an obliging friend or impatient spouse or child to do it for you. You can get accurate-enough measurements wielding your own tape measure.
The measurements you need to take might include:
Chest width: the distance across the chest at the underarm, above the bust. This is one of the most important measurements for the entire sweater. If the chest and the shoulders fit, the sweater will look and feel good. If the sweater is wider than the chest width, it will look sloppy. If the sweater is narrower than the chest width, it will pull across the shoulders. (On me: 13")
Back of neck: the distance across the back of the neck of the sweater, from the point where neck definitively turns into shoulder on each side. This is wider than the actual neck of the person. If you can't visualize this, measure across the back neck of a top that fits well. (On me: 5.5-6.5")
Shoulder width: the distance from the edge of the neck to the most sticky-up bone at the start of the shoulder. This does not include the ball of the shoulder, which is covered by the sleeve, just the part of the shoulder that will be covered by the body of the sweater. (On me, 3-3.5")
Slope of shoulder: the amount that the shoulders drop vertically in their run. Run a tape measure down the spine and eyeball a line to the top of the shoulder. (On me, about 2")
Underarm depth: the distance from the back neck to the true underarm. Run a tape down the spine and eyeball a line to the underarm. If measuring yourself, collarbone to middle of the armpit works pretty well. This is one of the most important measurements you can make. A lot of raglans place the underarm too deep, which leads to a sloppy fit. (On me, 7.5")
Underarm width: the distance across the armpit. This is used to determine how many stitches to cast on at the underarm. (On me, 3")
High chest: a tape wrapped tightly around the body at the underarms, above the bust. Don't skip this one if you want a good fit. This is a truer measure of yoke size than the full bust. (On me, 33")
Full bust: a tape wrapped snugly around the fullest part of the chest, over the bra. A must measurement for women, but not for men or children. Not-so-busty women can skip this one, too. (On me, 37")
Rib cage: a tape snugly around the rib cage under the bust. Another measurement for women, but not men or children. (On me, 33")
Bust tip: the distance between left bust tip and right bust tip (pretty much between the nipples. Used on women to place bust short rows. (On me, 8.5").
Vertical bust: the distance that the bust lifts the front of the sweater. You can either measure this by stretching a tape over the bust, stretching a tape over the same distance down the side of the body, and taking the different between the two (good for body-skimming sweaters) or wearing a loose t-shirt and measuring the actual difference between length of the part over the bust and the part on the side of the body. Also an important measure for those with pot bellies, since a pot belly can lift the front of a sweater much the same way a bust does. (On me, about 2")
Waist: wrap a tape snugly around the natural waist. Again, a measurement for women but not so important for slim men and children. Also a useful measurement for anyone with a pot belly.(On me, 30")
High hip: wrap a tape around the top of the hips, right over the hip bone. This is often a flattering length for women. (On me, 35")
Full hip: wrap a tape around the fullest part of the hips. A must if you're making a tunic-length or thigh-length garment. (On me, 38")
Full bust length: run a tape down the spine and measure to the full bust. This is likely to be a shaping maximum. (On me, 8")
Waist length: run a tape down the spine and measure to the natural waist. This is likely to be a shaping maximum or minimum spot. (On me, 16")
High hip length: run a tape down the spine and measure to the high hip. (On me, 20")
Full hip length: run a tape down the spine and measure to the fullest part of the hip. (On me, 24")
Body length: run a tape down the spine to the desired final sweater length.
Bicep: wrap a tape around the upper arm at the fullest point. (On me, 11")
Wrist: wrap a tape around the wrist. (On me, 6.5")
Sleeve length: run a tape from an inch below the underarm to the desired sleeve length for the sweater. You'll try on the sleeves anyway as you work, but this will give you a basis for planning sleeve decreases.